queue depth is defined as "outstanding I/O" in Wiki, but I don't quite get how one would know how many outstanding I/Os there are and wouldn't be there fewer outstanding I/Os with a faster SSD?

Well that plays into it since a faster SSD can plow through its workload at a greater place. The I/O speed and latency actually play hand in hand when measuring queue depth speeds. Higher queue depth to a certain point the I/O speed will remain the same but the latency will skyrocket.